OK so you put an empty brood box below the affected colony without a floor in it ... that sounds like a good idea ...just moves the big hole where the floor was down a bit further and that I suspect would make it less noticeable to robbers.
What about if you made a V-shaped smooth metal insert to go in the lower brood box with just a slot at the bottom so the dead bees would slide down through the slot into a container that the bees would not bother clearing out ...
I'm thinking that would be another nice little earner for my retirement ... the Argyle CBPV Floor !